You can monitor the progress in the Admin Area > Monitoring > Background Jobs page.
There is a specific Queue you can watch to see how long it will take to finish:
hashed_storage:hashed_storage_project_migrate

After it reaches zero, you can confirm every project has been migrated by running the commands bellow.
If you find it necessary, you can run this migration script again to schedule missing projects.

Any error or warning will be logged in Sidekiq's log file.

You only need the gitlab:storage:migrate_to_hashed rake task to migrate your repositories, but we have additional
commands below that helps you inspect projects and attachments in both legacy and hashed storage.

Rollback from Hashed storage to Legacy storage

If you need to rollback the storage migration for any reason, you can follow the steps described here.

NOTE: Note: Hashed Storage will be required in future version of GitLab.

To prevent new projects from being created in the Hashed storage,
you need to undo the enable hashed storage changes.

This task will schedule all your existing projects and associated attachments to be rolled back to the
Legacy storage type.

You can monitor the progress in the Admin Area > Monitoring > Background Jobs page.
On the Queues tab, you can watch the hashed_storage:hashed_storage_project_rollback queue to see how long the process will take to finish.

After it reaches zero, you can confirm every project has been rolled back by running the commands bellow.
If some projects weren't rolled back, you can run this rollback script again to schedule further rollbacks.